The study – conducted by researchers from the Johns Hopkins Bloomberg School of Public Health in Baltimore, MD – is published in the International Journal of Obesity.

According to the American Heart Association (AHA), the prevalence of obesity in children has more than tripled from 1971-2011. As a result of this high incidence among children, they are facing a wide range of health problems that were not seen until adulthood in older generations.

Some of these health problems include high blood pressure, type 2 diabetes and elevated cholesterol levels. Additionally, obese children are more likely to experience low self-esteem, depression and negative body image.

In the past, scientists working with penicillin discovered early on that its byproducts could cause weight gain in animals, paving the way for modern industrial farming practices of adding antibiotics to animal feed to fatten them up in a short span of time.

Though early-life antibiotic use has been previously linked to weight gain, until this latest study, there had not been any large-scale, population-based, longitudinal studies including a full age range of healthy children.

To conduct their study, lead author Dr. Brian S. Schwartz – a professor in the Department of Environmental Health Sciences at the Bloomberg School – and colleagues assessed electronic health records of 163,820 children between the ages of 3-18 years, from 2001-2012.

‘Systematic antibiotics should be avoided’

From the data, the team analyzed body weight and height to determine body mass index (BMI), and they also ascertained antibiotic use in the previous year.

Fast facts about childhood obesity in the US

Childhood obesity has doubled in children and quadrupled in adolescents in the past 30 years

In 2012, more than one third of children and adolescents were overweight or obese

Childhood obesity has immediate and long-term effects on health and well-being.

Results showed that at age 15, the children who had taken antibiotics seven times or more during childhood weighed around 3 lbs more than those who did not receive any antibiotics.

Interestingly, the researchers found that nearly 21% of the children in the study – almost 30,000 – had received seven or more antibiotic prescriptions during childhood.

Although Dr. Schwartz believes physicians are becoming more cautious in how often they prescribe antibiotics, he says parents often demand antibiotics for cold viruses and other conditions that will not benefit from antibiotic use.

What is more, concerns are growing that excessive antibiotic use is leading to the rise of antibiotic-resistant bacteria.

“Systematic antibiotics should be avoided except when strongly indicated,” says Dr. Schwartz. “From everything we are learning, it is more important than ever for physicians to be the gatekeepers and keep their young patients from getting drugs that not only won’t help them but may hurt them in the long run.”

Antibiotic use may change gut bacteria, leading to weight gain

Though the findings are substantial, Dr. Schwartz says the weight gain observed in the children who frequently used antibiotics is likely underestimated, as their lifetime antibiotic histories – including antibiotic use outside the health system – would not have been recorded.

“Your BMI may be forever altered by the antibiotics you take as a child,” says Dr. Schwartz. “Our data suggest that every time we give an antibiotic to kids, they gain weight faster over time.”

The researchers add that there is growing evidence antibiotics lead to weight gain in humans because of the effect they have on the microorganisms that reside in our bodies.

They explain that there are 10 times more bacterial cells in the human body than our own cells, and many of these bacteria work in the gastrointestinal tract to help the body digest food and absorb nutrients.

In doing their job of killing off harmful bacteria, however, antibiotics also kill off the “good bacteria.” Over time, repeated antibiotic use could change the microorganisms, affecting the way they break down food and increasing caloric intake – which can increase weight gain.